1. Field of the Invention
This invention relates to an apparatus for processing semiconductor wafers, and in particular, to an apparatus for thermal processing of a plurality of semiconductor wafers simultaneously.
2. Description of the Related Art
In doped-poly processes performed by the batch method, three wafers per batch are sampled; one from the top, one from the middle, and one from the bottom of the stack of wafers that were processed together in the batch. These wafers are referred to as monitor wafers. After each process, the monitor wafers for the batch are subjected to thermal processing and their resistances are measured.
In those situations where a specific thermal processing apparatus for semiconductor wafers is not available, diffusion process equipment is used instead. FIG. 1 shows a conventional approach for using a diffusion process apparatus as a thermal processing unit. In FIG. 1, a front shutter 3 is positioned on one side of a thermal processing unit 1 for loading a cassette containing wafers (not shown) onto a cassette stage 2. The cassette is then moved up by an auto cassette loader 4 to an upper cassette end 5 and held there.
The cassette at the upper cassette end 5 is subsequently moved down to a lower cassette end 6 by the auto cassette loader 4. A wafer transferer 7 takes the monitor wafers out of the cassette one at a time and transfers them to a boat 8. The boat 8, now loaded with monitor wafers, is moved to a heating chamber 10 by a boat elevator 9.
When the thermal processing on these monitor wafers is complete, the boat 8 retracts from the heating chamber 10 and is lowered by the boat elevator 9. Then, the monitor wafers are transferred one at a time from the boat 8 back to the cassette at the lower cassette end 6 by the wafer transferer 7. After this, the cassette is moved up to the upper cassette end 5 by the auto cassette loader 4 and finally back to the cassette stage 2 for removal of the cassette.
The conventional thermal processing unit has certain disadvantages, such as a complicated structure, and the processing time for one operating cycle is lengthy. In addition, during one operating cycle, the conventional thermal processing unit only performs the process on three monitor wafers from a single batch, which adds substantially to the total time for processing several batches. Even if the number of monitor wafers to be processed during one cycle were increased, the unit would be idle until the desired number of wafers are accumulated. This extended processing time also affects subsequent processing steps, and causes delays in the total semiconductor device fabrication process.